The present invention relates to a battery and a method for preparing the same. More particularly, the present invention relates to a battery, which has safety ensured by controlling temperature rise caused by short-circuit or the like and has improved battery characteristics such as discharge load characteristics and to a process for preparing the same.
Recently, with development in electronic appliances, high leveling of capacity and output density of a battery used as a power source is being advanced. As a battery which can satisfy these requirements, attention is paid to a lithium ion secondary battery. The lithium ion secondary battery has an advantageous effect that energy density is high, while a sufficient counterplan for safety is required because a non-aqueous electrolytic solution is used.
As a counterplan for safety it has been conventionally suggested to incorporate a safety valve which releases increased internal pressure, or a PTC device which increases resistance in accordance with the heat generated from external short circuit to break an electric current.
For example, as disclosed in Japanese Unexamined Patent Publication No. 328278/1992, there is known a method for attaching a safety valve and a PTC device to the positive electrode cap of a cylindrical battery. However, when the safety valve is operated, water in air may invade into a battery to react with lithium in the negative electrode and there is a fear of an exothermic reaction.
On the other hand, the PTC device successively breaks external short-circuit without causing any troubles. As a safety component running firstly at the emergency of the battery, the PTC device can be designed to run when the battery reaches at least 90xc2x0 C. due to external short circuit.
Since the conventional lithium secondary battery has the construction mentioned above, there exist the following problems.
At occurrence of short-circuit and temperature rise inside the lithium secondary battery, increase of the short-circuit current can not be controlled in a conventional lithium secondary battery.
When the short-circuit inside the lithium secondary battery increases a temperature, a polyethylene or polypropylene separator disposed between the positive electrode and the negative electrode is expected to have a function that the separator softens or melts to close holes thereon and release or confine a non-aqueous electrolyte contained therein to decrease its ion conductivity, and thereby reducing the short-circuit current.
But a separator away from the heating part does not always melt. Also, when a temperature further rises, the separator melts and is fluidized, and thereby the function to electrically insulate the positive electrode and the negative electrode is lost to cause short-circuit.
Besides, particularly in a lithium ion secondary battery, a negative electrode is formed by applying a slurry comprising a negative electrode active material such as graphite, a binder such as PVDF (poly(vinylidene fluoride)) and a solvent, onto a base material such as a copper foil which forms a collector, and drying it to form a thin film thereof. A positive electrode is also formed by applying a slurry comprising a positive electrode active material such as LiCoO2, a binder and a conductive agent onto a base material such as an aluminum foil which forms a collector to form a thin film thereof in the same manner.
The conductive agent is used to increase an electronic conductivity at a positive electrode when the positive electrode active material has insufficient electronic conductivity. As the conductive agent, there is used carbon black (such as acetylene black) or graphite (such as artificial graphite KS-6 available form LONZA Co., Ltd.).
Such a battery has a problem that when a temperature of the battery increases to at least a temperature that a separator melts and is fluidized due to internal short-circuit or the like as mentioned above, large short-circuit current flows between a positive electrode and a negative electrode at an area where the separator is fluidized, and thus temperature of the battery further increases due to the generation of heat, leading to a further increase of short-circuit current.
Japanese Unexamined Patent Publication No. 338240/1996 discloses a method for adhering an electrode, a separator and the like by using an adhesive agent in order to maintain the contact state of a contact interface between the electrode and the separator, when an aluminum laminate pack is used as a casing material in order to lighten the battery and enhance energy density. However, this method has a problem that when it is attempted to make an adhesive strength larger according to this method, the adhesive agent intrudes into an opening part of the separator to decrease an opening area and a hole diameter of the separator, and thus undesirable discharge load characteristics of a battery are caused.
The present invention has been carried out in order to solve the above problems. The object of the present invention is to provide a battery having an electrode capable of controlling increase of short-circuit current even at temperature rise due to generation of heat and having excellent discharge load characteristics.
The first battery of the present invention comprises an active material layer having an active material and an electronically conductive material contacted to the active material, and an electrolytic layer jointed with the active material layer, wherein the above electronically conductive material contains an electrically conductive filler and a resin so that resistance can be increased with temperature rise, and wherein the above active material layer and the electrolytic layer are jointed to each other by thermal treatment using the resin of the above electronically conductive material. According to this, the above electronically conductive material contains the electrically conductive filler and the resin to increase resistance thereof with temperature rise, and thus increase of current can be controlled. Furthermore, since the active material layer and the electrolytic layer are jointed to each other by thermal treatment by using the resin of the electronically conductive material, the extra adhesive agent is not needed, discharge load characteristics can be improved and high current can be provided.
The second battery of the present invention is that in the first battery, the resin is a crystalline resin or contains a crystalline resin. According to this, an increasing ratio of resistance with temperature rise (namely, changing ratio of resistance) can be improved by containing the crystalline resin in the resin, and there is obtained a battery capable of rapidly controlling increase of current flowing into the electrode.
The third battery of the present invention is that in the first battery, a melting point of the resin of the electronically conductive material T1 and a thermal treating temperature T2 satisfy T1xe2x89xa7T2. According to this, jointing is possible without changing properties of the electronically conductive material.
The fourth battery of the present invention is that in the first battery, a melting point of the resin is in a range of 90xc2x0 C. to 160xc2x0 C. According to this, by using the resin having a melting point in the range of 90xc2x0 C. to 160xc2x0 C., the electronically conductive material can increase changing ratio of resistance at about a pre-determined temperature in the range of 90xc2x0 C. to 160xc2x0 C., and thus characteristics of battery and safety can be coexistent with each other.
The fifth battery of the present invention is that in the first battery, a thermal treating temperature is in a range of 90xc2x0 C. to 160xc2x0 C. According to this, by jointing the electrolytic layer and the active material layer at a temperature in the range of 90xc2x0 C. to 160xc2x0 C., contact resistance of the above thermally fused interface can be increased simultaneously at a temperature where resistance of the electronically conductive material is increased.
The sixth battery of the present invention is that 0.5 to 15 parts by weight of the electronically conductive material is contained in 100 parts by weight of the active material. According to this, by using a battery containing 0.5 to 15 parts by weight of the electronically conductive material in 100 parts by weight of the active material, it is possible to lower resistance of the electrode before changing ratio of resistance of the battery is increased, and jointing strength suitable for battery production and battery performance can be provided.
The seventh battery of the present invention is that in the first battery, an amount of the electrically conductive filler is 40 to 70 parts by weight in the electronically conductive material. According to this, by setting the amount of the electrically conductive filler to 40 to 70 parts by weight in the electronically conductive material, it is possible that changing ratio of resistance with temperature rise is increased while normal resistance is low, and that discharging capacitance of the battery is increased.
The eighth battery of the present invention is that in the first battery, the electronically conductive material has a particle size of 0.05 xcexcm to 100 xcexcm. According to this, by using the electronically conductive material having a particle size of 0.05 xcexcm to 100 xcexcm, there is obtained desirable normal resistance and discharging capacitance when the electrode is applied to the battery.
The ninth battery of the present invention is that a carbon material or an electrically conductive non-oxide is used as the electrically conductive filler. According to this, since the carbon material or the electrically conductive non-oxide is used as the electrically conductive filler, the electric conductivity of the electrode can be improved.
The tenth battery of the present invention is that in the first battery, the active material layer contains a conductive agent. According to this, since the electrode contains the conductive agent, resistance of the electrode can be suitably controlled even when the electronically conductive material having a small electronic conductivity is used.
The first process for preparing a battery of the present invention comprises the steps of:
(a) forming fine particles of the electronically conductive material by pulverizing an electronically conductive material comprising an electrically conductive filler and a resin;
(b) preparing a paste active material by dispersing the above fine particles of the electronically conductive material and an active material in a dispersion medium;
(c) forming an electrode by drying the above active material paste and by pressing it at a predetermined temperature T1 and a predetermined pressure; and
(d) layering the above electrode and the electrolytic layer and fusing it at a predetermined temperature T2 and a predetermined pressure.
According to this method, since it comprises the steps (a) to (d), the adhesion between the electronically conductive material and the active material becomes high, and thus resistance of the prepared electrode or resistance of interface between the electrode and the electrolytic layer can be decreased.
The second process for preparing a battery of the present invention is that in the first method, the resin is a crystalline resin or contains the crystalline resin. According to this, by containing the crystalline resin in the resin, a rate of increase in resistance to temperature rise (namely, changing ratio of resistance) can be improved, and there is obtained a battery capable of rapidly controlling increase of current flowing into the electrode.
The third process for preparing a battery of the present invention is that in the first method, predetermined temperatures T1 and T2 are a melting point of the resin or a temperature near the melting point. According to this, by setting the predetermined temperatures to the melting point of the resin or the temperature near the melting point, the adhesion between the electronically conductive material and the active material is further improved, and thus resistance of the prepared electrode or resistance of interface between the electrode and the electrolyte maintaining layer can be further decreased.